As semiconductor laser driving methods, a zero-bias driving method and bias driving method have conventionally been used. According to the zero-bias driving method, the bias current of a semiconductor laser is set to 0, and the laser is driven by a pulse current corresponding to an input signal. According to the bias driving method, the bias current of a semiconductor laser is set to a predetermined threshold current, and the laser is driven by adding a pulse current corresponding to an input signal to a bias current while always supplying the bias current.
When a semiconductor laser is activated from a zero-bias state, an emission delay generally occurs because a given time is taken until carriers of a concentration enough to oscillate a laser are generated after a drive current corresponding to an input signal is applied to the semiconductor laser. When the semiconductor laser is driven at a high speed, only a pulse whose width is smaller than a desired one can be obtained.
In order to shorten the delay time till laser oscillation, there is proposed a method of supplying an oscillation threshold current as a bias current to a semiconductor laser in advance. In particular, recent laser printers, digital copying machines, DVD apparatuses, and the like require higher image qualities, and systems using a 650-nm red semiconductor laser and 400-nm blue-violet semiconductor laser have come into practical use.
The red semiconductor laser applied to a laser printer, digital copying machine, or the like emits an optical output of about several ten μW while a bias current is supplied. The influence of ground stain (fog) by the bias current can be ignored.
To the contrary, some of blue-violet semiconductor lasers emit an optical output of about 1 mW during supply of the bias current, and the optical output is higher than that of the red semiconductor laser. A blue-violet semiconductor laser having a narrow far-field pattern can expose a photosensitive material by an optical output of about 3 mW. Hence, the difference between the exposure potential and the non-exposure potential on the photosensitive material becomes smaller than a conventional one, and the development bias setting range of whether to apply toner also becomes narrower. When the blue-violet semiconductor laser is applied to a laser printer, digital copying machine, or the like, ground stain (fog) occurs owing to variations in laser emission characteristic.
The blue-violet semiconductor laser has a short wavelength, and its laser spot can be easily narrowed down in comparison with the red semiconductor laser. However, the voltage drop by the blue-violet semiconductor laser is large, and power consumption of the laser increases. Large power consumption raises the temperature and shortens the service life of the laser itself.
When the blue-violet semiconductor laser is used in a laser printer, digital copying machine, or the like, application of the zero-bias driving method is examined. In the optical communication field, there is proposed an arrangement in which the zero-bias driving method is basically used and an oscillation threshold current (bias current) is supplied immediately before a drive current for causing a semiconductor laser to emit light is supplied (e.g., prior art reference 1: Japanese Patent Laid-Open No. 4-283978, and prior art reference 2: Japanese Patent Laid-Open No. 9-83050). In these proposals, a delay circuit which delays an input signal by a predetermined time is arranged, and the drive current is set “ON” by an output from the delay circuit.
However, the following problems arise when the arrangements in prior art references 1 and 2 are applied to an image forming apparatus such as a laser printer or digital copying machine.
Experimental results reveal that, to activate a semiconductor laser after a long laser OFF period, the laser can be generally stably activated after a bias current is supplied for at least several nsec. For image data for which the laser OFF period of the semiconductor laser is short, the semiconductor laser can respond only by supplying a bias current for a period shorter than the above-mentioned period because of the influence of residual charges of a driver and the characteristics of the semiconductor laser.
In prior art references 1 and 2, the semiconductor laser is driven after a delay of a predetermined time from an input signal. For example, the bias current keeps flowing even during the OFF period for image data (high-density image) for which the OFF period t is shorter than the delay time τ, as shown in FIG. 11. Weak exposure is undesirably caused by the bias current during the OFF period, i.e., at a white pixel, and toner is applied to the white pixel.
To prevent these problems, when the arrangements in prior art references 1 and 2 are applied to an image forming apparatus, the timing at which the bias current is supplied is desirably generated not by delaying image data but by controlling the ON timing of the bias current.
If the laser is driven with a delay of a predetermined time from an input signal, like prior art references 1 and 2, the image write position shifts by the delay time, and thus must be controlled in advance in consideration of the delay time. In order to generate a very short delay time, the delay circuit requires a high-precision element excellent in temperature characteristic, and raises the cost in comparison with a conventional laser drive circuit. In order to control the image write position, a precise delay amount must be fed back to the write control system, and control of the image write position becomes complicated.
When a delay circuit which delays an input signal is formed from a multistage buffer IC or the like, delayed outputs slightly vary in time owing to the threshold voltage in the buffer IC. Such variations do not matter in the optical communication field. However, in a high-quality digital copying machine and printer, the variation width generates a high-frequency jitter, and the jitter image appears in the sub-scanning direction, degrading the image quality.